This invention relates in general to a method and control device for preventing register errors caused by variable contact forces of a pulley gripping a conveyor belt.
When printing stock, for example, a sheet of paper or similar material, by printers, the correctly positioned printing of the print image on the stock is of considerable importance. This characteristic is designated by the term registerability. To ensure the registerability, register marks are used outside the printing image, by which deviations from the correctly positioned printing are captured and measured by the operator of the printer. With further development of this method, the registerability is determined by sensors in the printer and a possible register error calculated. To this end, the sensors detect the register marks on the conveyor belt or stock and use the position of the register marks to determine whether the printing is taking place flawlessly or not.
The method and devices of the state-of-the-art technology capture and correct register errors, which are caused, for example, by mechanical shifting of the stock on the conveyor belt, by changes in speed of the conveyor belt or the printing drum, or by changes in the thermal surface on the impression drum and the resulting transmission errors between the illustration drum and the printing drum. The paths traveled by the conveyor belt carrying the stock, according to which the image is applied to the stock, are, however, determined by a certain time-lag that elapses during the movement of the stock onto the conveyor belt between a sensor signal, or a signal derived therefrom at the beginning of the printing module of the printer, and a printing gap or nip on a printing module, which applies the image to the stock. Likewise, the image path traveled from the illustration device, during which a latent electrostatic image is applied to an illustration drum, to the printing gap or nip between the printing drum and the conveyor belt, is determined by a specified time. Due to the previously described influences, the preset time lags specified in one control device of the printer are erroneous. The printing image, due to the existence of the changes to the printing drum in the printing module is applied to the stock in a shifted position. This leads to a register error.
Another reason for the register error is due to the fact that the contact force of the snub pulley affects the rotational velocity of the printing drum on the opposite side of the conveyor belt. If an intermediate drum is used, which is attached by friction to the printing drum, the rotational velocity of the printing drum is correspondingly affected. As a result of the effect of the rotational velocity of the printing drum, the time lag at which an image frame or frame of the printing drum is applied to the stock is changed. For example, an image frame or frame is delayed on the stock, if the rotational velocity of the printing drum is reduced. Another reason for register errors is the pressing of a snub pulley gripping the conveyor belt below into the printing gap or nip, which, as a result of the variable contact force, changes, as described below. The snub pulley provides a counterforce to the printing drum above the conveyor belt. The forces of the printing drum on the stock or conveyor belt is required to mechanically transfer the toner from the printing drum onto the stock and to transfer the toner image subsequently. Furthermore, the toner image, which consists of register marks in this case, are sometimes transferred to the conveyor belt, as in the case of the calibration run of the printer. The contact force of the printing drum has an affect on the resolution of the image lines that compose an image. The higher the contact force of the printing drum, the further the image lines move apart, as described in detail below. In the printing gap, or nip, errors are incurred by the distance of the image lines from one another. Both of the last-named effects are designated and considered as the first register error and as the second register error in the existing description.
It is the purpose of this invention to correct register errors caused by variable contact forces of a snub pulley. According to the invention, the register errors are caused by the variable contact force of a snub pulley gripping the conveyor belt, and to prevent a first register error, the time-lags of the first start signals (START OF FRAME) for the application of images or frame are changed and to prevent a second register error by two start signals (START OF THE LINE), time-lags for the application of image lines are changed. Furthermore, an illustration device is provided for the transfer of image lines to a printing drum, with a first sensor for detecting a stock in front of the printing modules, a second sensor to detect the register marks behind the printing modules, a rotary encoder to capture the rotation angle of an illustration drum and a device to store the values of the first start signals (START OF FRAME) for the application of image frames or frames and of second start signals (START OF LINE) for the application of image lines, which are determined by the variable contact force of a snub pulley gripping the conveyor belt.
In a particularly advantageous manner, the start signals are adjusted to the case if stock is located between the printing drum and the conveyor belt. In this case, the contact force of the printing drum and consequently the register error change in particularly forceful ways. Furthermore, the start signals may be related to the properties of a stock. In this manner, the distinct change of the image line definition, i.e., the distance of the image lines on the stock from one another, variable contact forces are taken into consideration with different stock. The contact force of the snub pulley thus increases constantly if the snub pulley is shifted. In this case, increasing forces of the pneumatic position of the snub pulley work against such force. For example, a variable contact force of the snub pulley has less of an effect with a strongly compromised stock than with a slightly compromising stock, since the snub pulley is not so strongly deflected as with a slightly compromising stock. The greater the deflection of the snub pulley, the greater the contact force. With a strongly comprising stock, the image lines do not move as far apart as with a slightly compromising stock, if the contact force of the snub pulley and consequently the force of the printing drum on the stock increases. The contact force of the snub pulley depends upon the contact force of the printing drum, since the snub pulley is arranged opposite the printing drum, whose forces work against one another. In addition, the thickness of the stock is taken into consideration, which affects the contact force of the snub pulley, since the contact force is proportional to the path to which the snub pulley is shifted due to the stock in the printing gap or nip.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.